The long-term goal of these studies is to understand how the sensitivity of the visual system is preserved during transmission through the retina. From behavioral and physiological studies it is known that the dark-adapted visual system is capable of transducing and transmitting the absorption of even a single photon of light to the brain. While the mechanisms responsible for the reliable transduction of single photons are now well understood, much less is known about how these small signals are faithfully transmitted through the retina. By performing paired patch-clamp recordings in retinal slices and Ca2+ imaging on isolated rods, two synaptic mechanisms likely to play a role in preserving these signals during transmission from rods to second- order retinal cells will be examined: Ca2+ dynamics in the rod terminal and transmitter release from the rod terminal. This work will contribute to a general understanding of the mechanisms that assure that the visual system operates at the peak of its potential.